This ATO has saved me from carrying heavy 5 Gal buckets back and forth across the house, and the two different sensor technologies give me a sense of security when it come to functioning properly and not becoming being stuck in the on position and overdosing kalk!

I found this top off unit easy to set up and comes with everything you need(no extra pump or valve needed). I would highly suggest this product as the first choice. The only drawback I found is when installing the magnet sensor mount in the tank (instead of a sump) I found the trim along the top of the tank will stop the outer magnet from going as high up on the tank as I wanted but, the remedy is as easy as a clamp or tank silicone adhesive. I am using an old 5 gallon salt bucket for my top off water. I added a float valve and enough 1/4" line to reach the RO/DI system in the basement so I dont have to hall the top off water up 2 flights of stairs to fill the bucket. I installed a ball valve on this line and only turn it on when the bucket needs to be filled. Works great!

I had some problems with this unit right off the bat...

Firstly, the pump didn't seem to be pumping anything. After several minutes of running (as evidenced by the whining noise), still no water was coming out. The ATO reservoir sits right beside the sump, so it only has to pump a few feet, and with about 14 inches of head. Finally, I primed the line using the age-old sucking technique, and from that point on it seems to be working.

The next, and bigger, problem was that the float switch, which is supposed to be the safety backup, was not working at all. Regardless of whether I lifted it manually or submerged it, no alarms would sound and the ATO kept pumping away. I contacted Roger at Tunze on their support forum at ReefCentral, and he confirmed that the magnets inside the float switch bracket were installed (by the factory) upside down. They are supposed to be at the bottom of the bracket, but these ones were at the top. Becuase the float switch itself operates on magnetic principles, the mis-aligned magnetic field was interfering with the switch, causing it to fail. After a botched attempt at fixing it myself, Roger is sending me a replacement bracket. No hassles--just a simple replacement.

The Osmolator and I still have yet to build a trust relationship. I've decided to plug it into one of my Apex controller's switched outlets, with yet another float switch (monitored by Apex). This way, the Apex can also kill the power to the top off if it detects levels getting too high. Redundancy is key.

The only other gripes I have with it, as other people have mentioned, are that it automatically runs the pump for several seconds when initially powered up (why?) and that the pump itself has a high-pitched shrill whiney sound to it, so it can easily be heard from outside the stand when it's running.

In general, after much manual testing of the optical eye sensor, I would say that it works quite well, and I am pleased with it. The price is hard to swallow, but I rent my apartment and it has hardwood floors, so I'm willing to take all precautions necessary to prevent overflow disasters.

Tunze customer service (Roger) has been very responsive, so in that regard, I give them (him) an A++.

This is a very nice ATO that keeps the level of water right where you want it. It saves me about 15 min a day at least from manually having to refill my sump, that's a lot of time saved after a while. well worth the cost, quality unit and you can tell a whole lot of thought went into this unit.

Love this system but the only flaw is the pump does not last, after 3 months it starts to drips after pump is off and sometime its a non stop drip. I'm on my 3 pump now and the system is only 9 months old. But other than that its a great ATO sytem, I just keep a extra pump now.

I just ordered my second Osmolator as the control module of my first one just died after 3 years. I didn't consider buying any other ATO system as this thing worked perfectly until it went out. The Osmolator is so precise at maintaining water level which is essential for growing healthy coral.

I like to have my 90 gallon reef tank as full as possible so it took a little time to adjust the over flow shutoff switch to the correct level. Once the sensors were adjusted to the correct height it has worked flawlessly. I'm very pleased with the Tunze 3155 Osmolator. It's worth its weight in gold.

This is a good product that works but has some flaws:

1. Every time the unit loses power or is unplugged and plugged in again, it will start pumping water for 10-15 seconds even if you are at the correct level or above.
2. There is an alarm to signial too much water. It's loud and can not be disabled unless you take the unit apart and move a jumper.
3. The eye didn't work correctly out of the box. I had to wet it and rub it with a clean cloth vigorously to get it to function. Once cleaned it worked. I question it's long term reliability.

Kinda wish I went with the JBJ ATO and saved 100 bucks.

Great product, set it and forget it. This product will make your life a lot easier.

I purchased this product to go with a Kalk stirrer I recently purchased from BRS. I had watched the videos from the knowledgeable staff at BRS. I have to say this auto top off is the best thing I ever purchased for my system. It is extremely easy to set up and you don't have the worry of it over filling since it comes with multiple safety features.

I was hesitant to buy this, given the premium price, but it keeps my sump level incredibly steady. MUCH better than the upside-down water cooler jug gravity feed setup I had before. The only odd thing about it is that it runs the pump for about 5 seconds or so any time you pull the plug and plug it back in. Not an issue unless you're turning it on and off a lot. Though that could happen if you have it on a controller set to only turn it on for say 5 minutes every hour. Just something to be aware of.

The Tunze osmolator is not the cheapest ATO, but I think it is the best. Worth the price for piece of mind.